1. Field of the Invention
The invention relates to an improved arrangement for testing long, installed cables which contain multiple conductors.
2. Description of the Prior Art
Electrical cables having multiple conductors, so-called multi-conductor cable, are extensively used in electronic systems to interconnect various electrical subassemblies or subsystems. Frequently, for example, in the interconnection of two telephone switching offices, an installer first runs multi-conductor cable between two locations and then attaches a connector to each end of the cable. These connectors mate with corresponding connectors on the equipment which is to be interconnected by the cable.
Each connector is comprised of a plurality of pins or terminals. An installer attaches each pin, usually by soldering, to a respective conductor in the cable. During this procedure, wiring errors are often made. These wiring errors or conduction faults typically include: open conductors, i.e., those which are physically, but not electrically, attached to corresponding pins in the connector; conductors shorted to others, illustratively caused by solder bridging between a number of adjacent pins, and crossed conductors, i.e., those which are wired to wrong pins on the connector. These errors, if left uncorrected, can, by applying a signal to a wrong pin, seriously damage any equipment to which the cable is connected.
To accurately test a cable for wiring errors, every conductor must be tested against every other conductor. One traditional technique for doing this involves "ringing out" the cable until all the errors are found. In particular, once both connectors have been attached to the cable, an installer then connects one probe of an ohmmeter, buzzer or similar test instrument, to a pin of one connector. The installer then successively touches the other probe of the test instrument to every other pin in the same connector to see if the instrument indicates current flow and thus indicates a wiring error involving that first pin. Thereafter, the installer moves the first probe to another pin and repeats the entire procedure. Since a multi-conductor cable having as little as 10 to 20 conductors may require more than several thousand separate tests, and because a good deal of time is required to move the probes from pin to pin, this technique wastes a considerable amount of time.
Various suggestions have been put forth in the art to substantially reduce the time required to test a multi-conductor cable. However, each suggestions has its own peculiar disadvantages if it is used to test a long, installed multi-conductor cable.
For example, U.S. Pat. No. 2,904,750 (issued on Sept. 15, 1959 to E. P. Gargani et al) discloses an arrangement which tests a multi-conductor cable by applying a voltage level to one end of a selected conductor and, if detected at the other end, using that level to advance the position of a relay armature such that that level is applied to another conductor. The relay stops advancing whenever a wiring error is found. Unfortunately, with this arrangement, both ends of the cable under test must be brought to a single location. Hence, this arrangement cannot be used if both ends of an installed cable are separated by a large distance.
Another suggestion is typified by the arrangement shown in U.S. Pat. No. 2,964,701 (issued on Dec. 13, 1960 to M. L. Argabright). With this arrangement, an adapter, attached to one end of the cable, connects each of the conductors in the cable in series. The other end of the cable is connected, as one leg, to a four-leg balanced bridge network. A balance is achieved if none of the conductors is shorted together or crossed. While this arrangement eliminates the need to bring both connectors of the cable to a single testing location, this arrangement is not able to determine which particular conductors are incorrectly wired and the exact nature of the wiring error, i.e., whether it is an open, shorted or crossed conductor.
Another suggestion is the arrangement disclosed in U.S. Pat. No. 3,217,244 (issued on Nov. 9, 1965 to G. G. Glover). In this arrangement, an adapter is connected to one end, illustratively the far end, of the cable under test. This adapter contains as many separate indicators as there are conductors. Within this adapter, one side of an indicator, typically a lamp, is wired in series, via a mating connector, with a respective conductor in the cable. The other side of all the lamps is wired together. At the other end of the cable, i.e., the near end, this arrangement selects a particular conductor and applies test current to it in an amount sufficient to light a corresponding indicator in the adapter. All the remaining conductors in the cable are used as the return path for the lit indicator. If the returning test current is unevenly distributed among these remaining conductors, this may disadvantageously cause an erroneous indication. Specifically, if one conductor in the return path or its associated indicator has a low resistance with respect to the others, then the test current flowing through that conductor will be relatively high and that indicator in the return path may light. Since this indicator will light simultaneously with the one wired in series with the selected conductor, the installer will interpret this indication to mean that both conductors are shorted together--when in fact they are not.
A further solution is the arrangement disclosed in U.S. Pat. No. 3,825,824 (issued July 23, 1974 to R. G. Herron et al.). Here, the test current is applied at the near end to only one conductor at a time in an amount sufficient to light a corresponding indicator located at the far end. However, a separate source of power must be applied to the apparatus positioned at both ends of the cable. Moreover, the return path disadvantageously lies outside the cable. Consequently, if a long, installed multi-conductor telephone cable is to be tested, a conductor, located outside the cable and terminating at the same physical locations as both ends of the cable under test, must be found for use as the return path. "Free" conductors of this sort can rarely be found in telephone applications and oftentimes power supplies are not available at both ends of a long installed telephone cable. Thus, this apparatus can not be readily used to test typical long installed multi-conductor telephone cable.